This is the simplest machine structure you will see outside of an early minicomputer. It was an early classical machine, arithmetic was done in its single accumulator (A) and multiply quotient (Q) registers. That was the way everyone built computers in the beginning because registers were so expensive. There were six index or (B) registers. Data is transferred between the 1604 and IO (either a tape controller or the CDC 160) via 3 Input and 3 Output channels that operate concurrent with the main program. In addition there is a block transfer instruction for the high speed transfer of data. Program interrupt was used to synchronize the various IO transfers.